1. Field of the Invention
The invention relates to compositions and molded articles for formation of articles resistant to rapid gas decompression and resultant cracking and rupture.
2. Description of Related Art
Rapid gas decompression (RGD), also known as explosive decompression (ED) often occurs when high-pressure gas molecules migrate into an elastomer at a compressed state. When the pressure surrounding the elastomer is released suddenly, the compressed gas inside the elastomer tries to expand and exit the elastomer. This gas expansion can cause seal damage such as tears, holes, blisters and cracks.
Rapid gas decompression (RGD) resistant fluorocarbon elastomers are known and widely used in seals and gaskets, etc. and in end use sealing applications such as compressors and down hole applications in the oil industry where seals and other articles formed of such elastomers can encounter long soak times in high pressure gases such as carbon dioxide and methane. Leaks and excessive cracking in such applications can lead to unnecessary downtime associated with unforeseen maintenance. Unplanned maintenance can cause not only significant production time, but financial loss and additional potential incidents where safety hazards to workers may occur.
Standards such as NORSOK M710 and ISO 23936-2:2011(E), each is incorporated herein by reference, have been written to serve as an industry benchmarks for RGD resistance and to evaluate the performance of seals in rapid gas decompression environments. The NORSOK M-710 standard (Qualification of Nonmetallic Sealing Materials and Manufacturers) was introduced by the Norwegian petroleum industry to define the requirements for critical nonmetallic (polymer) sealing applications in conditions where failures can be costly from a safety and financial standpoint. The NORSOK M-710's criteria were developed to ensure safe and cost-effective solutions for sealing in the oil and gas industry. Similarly, the ISO rating is from the International Standards Organization. ISO 23936 provides principles, requirements and recommendations for selecting and qualifying polymer/elastomer materials for service in oil and gas production environments in typical equipment used in those industries. These standards are created to avoid failure of such materials in service which can present a risk to the health and safety or to the environment as well as losses in production to manufacturers.
The lower the rating number achieved when measuring seals performance under these standards, the lower the cracking under the standard test conditions and the higher the resistance of the material to RGD. For example, an ISO rating of 0 to 3 is considered passing for RGD use, wherein 0 indicates no cracks, holes or blisters and exposed surfaces remain intact after ISO exposure testing and a rating of 3 on the lower end of passing represents that there may be a number of cracks of which two internal cracks can each have a length of 50% to 80% of the cross sectional diameter of the test seal piece and a total crack length may not exceed three times the cross sectional diameter of the test seal piece. Further external cracks must be <50% of the cross sectional diameter of the test seal piece with no splits being permitted.
Whether seals and other elastomeric components suffer from RGD in terms of cracking or deterioration and pass or fail such industry standards is a function of working environment (exposure to harsh materials, pressure or temperature) and material of construction. While the former is difficult to alter in a given end use, the material may be modified in the art to create better products. Factors which affect the material in an RGD use include its formulation, manufacturing, size and contamination resistance among other factors. Continued exposure to high temperatures, fast depressurization rates and numerous cycling can impact the severity of damage to RGD-resistant materials, and consequently, how long they last in use and their tendency to require emergency replacement should a seal or other component rupture or breakdown in use.
In addition to making sealing and other elastomeric components more RGD-resistant, it is also a goal in the art to try to maintain generally good elastomeric properties such as good processability, low shrinkage and, in some applications, lower hardness.
RGD-resistant seals are known in the art. One such seal is commercially available from Greene, Tweed & Co., Inc. of Kulpsville, Pa. as FKM 938. This material satisfies a passing rating under the ISO standard, typically achieving ISO ratings of 1 or 2 depending on end use. Similarly, E. I. DuPont de Nemours offers RGD-resistant compounds including those commercially available based on Viton® Extreme™ ETP-S and Viton® GF-S which are promoted as having its best ED performance in a 100% carbon dioxide environment and as satisfying a passing rating of 0-3 in compressed and uncompressed states.
Another problem encountered in the art, besides continuing to improve the RGD resistance and cracking issues when a seal or other component is placed in service, includes the need to increase the size of the parts exposed to such environments for variety in component design without losing RGD resistance. It is known in the art that smaller seals can perform well over a range of decompression conditions if otherwise identified as passing. However, when cross sectional diameters become greater than about 5.33 mm, and as they approach 10 mm or even greater, then can begin to perform generally badly, even if one is using an RGD-resistant elastomer material. Thus, when larger seals are needed, it has been the case that one needs to redesign an end application to use a smaller seal, or must move to a different material such as a sprung PTFE seal. See, E. Ho, “Elastomeric Seals For Rapid Gas Decompression Applications in High-Pressure Services,” BHR Group Limited for the Health and Safety Executive, Research Report No. 485, pp. 36 (2006).
While such compounds are available, there is still a need in the art to continue to improve RGD-resistance in harsh environments, as such environments become more demanding, and to remain RGD resistant for longer periods of time and/or under harsher conditions. Further, it would be advantageous if larger sized seals could be formed from such materials without losing RGD resistance. Therefore, a need in the art exists for a composition that can repeatedly satisfy the industry standards, while having high passing ratings when in service to improve service life, reduce maintenance downtime in production, and to protect worker safety as well as the environment.